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Magnetic Anisotropy of Polycrystalline High-Temperature Ferromagnetic MARK Mnxsi1-X (X ≈0.5) Alloy films ⁎ A.B

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Magnetic Anisotropy of Polycrystalline High-Temperature Ferromagnetic MARK Mnxsi1-X (X ≈0.5) Alloy films ⁎ A.B Journal of Magnetism and Magnetic Materials 429 (2017) 305–313 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Magnetic anisotropy of polycrystalline high-temperature ferromagnetic MARK MnxSi1-x (x ≈0.5) alloy films ⁎ A.B. Drovosekova, , N.M. Kreinesa, A.O. Savitskya, S.V. Kapelnitskyb,c, V.V. Rylkovb,d, V.V. Tugushevb,e, G.V. Prutskovb, O.A. Novodvorskiif, A.V. Shorokhovaf, Y. Wangg, S. Zhoug a P.L.Kapitza Institute for Physical Problems RAS, Kosygina St. 2, 119334 Moscow, Russia b National Research Centre “Kurchatov Institute”, Kurchatov Sq. 1, 123182 Moscow, Russia c Institute of Physics and Technology RAS, Nakhimovsky av. 36, build 1, 117218 Moscow, Russia d Kotel'nikov Institute of Radio Engineering and Electronics RAS, 141190 Fryazino, Moscow Region, Russia e Prokhorov General Physics Institute RAS, Vavilov St. 38, 119991 Moscow, Russia f Institute on Laser and Information Technologies RAS, Svyatoozerskaya St. 1, 140700 Shatura, Moscow Region, Russia g Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstrasse 400, 01328 Dresden, Germany ARTICLE INFO ABSTRACT Keywords: A set of thin film MnxSi1-x alloy samples with different manganese concentration x ≈0.44−0.63 grown by the fi Magnetic MnxSi1-x alloy lms pulsed laser deposition (PLD) method onto the Al2O3 (0001) substrate was investigated in the temperature Magnetic anisotropy range 4–300 K using ferromagnetic resonance (FMR) measurements in the wide range of frequencies Ferromagnetic resonance ( f =7−60GHz) and magnetic fields (H =0−30kOe). For samples with x ≈0.52−0.55, FMR data show clear evidence of ferromagnetism with high Curie temperatures TC ∼ 300 K. These samples demonstrate a complex magnetic anisotropy described phenomenologically as a combination of the essential second order easy plane anisotropy contribution and the additional fourth order uniaxial anisotropy contribution with easy direction normal to the film plane. The observed anisotropy is attributed to a polycrystalline (mosaic) structure of the films caused by the film-substrate lattice mismatch. The existence of extra strains at the crystallite boundaries initiates a random distribution of local in-plane anisotropy axes in the samples. As a result, the symmetry of the net magnetic anisotropy is axial with the symmetry axis normal to the film plane. The principal features of the observed anisotropy are explained qualitatively within the proposed microscopic model. 1. Introduction materials in spintronic applications. At the same time, the fabrication of well-reproducible homogeneous magnetic alloys with high Mn Development of Si based magnetic semiconductor materials for content x ≈0.35 is difficult because of the variety of stable phases of spintronic applications attracts a lot of attention, since these materials high MnSiy silicides (not less than five) with the close content of can be easily incorporated into the existing microelectronic technology components (y = 1.72 − 1.75) [6,7]. [1]. In particular, Si-Mn alloys demonstrating unusual magnetic and In contrast, nonstoichiometric MnxSi1-x alloys with high Mn transport properties [2–8] have especial interest to engineer non- content (x ≈0.5, i.e. close to stoichiometric MnSi) are not inclined to conventional integrated-circuit elements. a phase segregation and formation of isolated magnetic precipitates, so However, there exist significant technological and fundamental they seem more promising for spintronic applications than dilute obstacles to adapt Si-Mn based elements to the needs of spintronic. MnxSi1-x alloys. Recently we have found that in thin films of such At relatively low Mn content in MnxSi1-x alloys (x = 0.05 − 0.1), the concentrated alloys, the Curie temperature TC increases by more than ferromagnetism (FM) at above room temperature has been revealed. an order of magnitude as compared with bulk MnSi (TC ≈30K) [6]. But these alloys turn out to be strongly inhomogeneous materials due Comparative studies of anomalous Hall effect and transverse Kerr to their phase segregation, leading to formation of isolated magnetic effect showed that the ferromagnetic transition in MnxSi1-x MnSi1.7 precipitate nanoparticles with the Mn content x ≈0.35 in Si (x ≈ 0.52 − 0.55) alloys occurring at T ∼ 300 K, has a global nature ff matrix [2]. In such alloys, anomalous Hall e ect testifying the spin and is not associated with the phase segregation [7]. Besides high TC polarization of carriers is absent, that makes impossible to use these values, the films investigated in [6,7] show large values of saturation ⁎ Corresponding author. E-mail address: [email protected] (A.B. Drovosekov). http://dx.doi.org/10.1016/j.jmmm.2017.01.022 Received 22 September 2015; Received in revised form 21 December 2016; Accepted 8 January 2017 Available online 10 January 2017 0304-8853/ © 2017 Elsevier B.V. All rights reserved. A.B. Drovosekov et al. Journal of Magnetism and Magnetic Materials 429 (2017) 305–313 magnetization reaching ≈400 emu/cm3 at low temperatures. The fi observed magnetization value corresponds to ≈1.1μB /Mn, that signi - cantly exceeds the value 0.4μB /Mn typical for bulk MnSi crystal [9]. High temperature FM in MnxSi1-x (x ≈0.5) alloys has been qualitatively interpreted [6,7] in frame of the early proposed model for dilute MnxSi1-x alloys [3], i.e. in terms of complex defects with local magnetic moments embedded into the matrix of itinerant FM. However, many details of FM order in MnxSi1-x(x ≈0.5) alloys are still not completely clear due to insufficient experimental studies. In particular, there are no data on their magnetic anisotropy features. In the present work, thin MnxSi1-x (x ≈ 0.52 − 0.55) films are investigated by the ferromagnetic resonance (FMR) method which is powerful for providing valuable information about magnetic anisotropy peculiarities of thin film magnetic materials, in particular, like dilute magnetic semiconductors (see [10] and references therein). Our studies are largely focused on the position and shape of FMR signal, while the analysis of the refinements of the line width data providing additional information on the magnetic inhomogeneity and relaxation rate of fi Fig. 1. Temperature dependence of the resonance eld HTres( ) at the frequency magnetization is not presented in this work. Besides, to study the 17.4 GHz for samples with different manganese concentration. The dashed line details of crystalline and magnetic microstructures of our films we corresponds to the calculated position of paramagnetic resonance for g-factor g=2.14. perform in this work the atomic force microscopy (AFM) and magnetic The inset demonstrates examples of the experimental resonance spectra at T=77 K. The force microscopy (MFM) investigations. We hope that AFM and MFM field is applied in the film plane. methods allow for additional understanding of the FMR results, since these methods are able to reveal the “local” effects of the crystal and temperature increases, the resonance field also increases and at magnetic texture of the film on the origin of magnetic anisotropy T ∼ 300 K reaches the value corresponding to the paramagnetic established in FMR measurements. resonance situation. The observed absorption lines have the Lorentz- like shape, and the resonance field anisotropy in the film plane is 2. Samples and experimental details absent. The shift of the FMR absorption line revealed in our measurements We studied six samples with manganese content in the range (shown in Fig. 1) clearly indicates the presence of FM moment in x ≈ 0.44 − 0.63. The 70 nm thick film samples were produced by the samples with x ≈ 0.52 − 0.55 at low temperatures, in agreement with Ref. [6]. When the external magnetic field H is applied in the plane of a pulse laser deposition (PLD) method on Al2O3 (0001) substrates at 340 °C. The composition of the films was testified by X-ray photoelec- thin FM film with magnetization M, the FMR frequency f may be tronic spectroscopy (for details see [6]). described by the phenomenological formula (see Appendix A): The structural properties of the samples were studied by X-ray f =(+)γHH KM. diffraction (XRD) analysis using a Rigaku SmartLab diffractometer (1) without monochromators and a diaphragm before the detector. In this Here K is the effective easy plane magnetic anisotropy coefficient; K M 9 case, intensity of the direct beam was as high as 1.5·10 pulses/s. is the effective field of easy plane magnetic anisotropy. This field Additionally, we performed room temperature AFM and MFM inves- describes the effect of two factors on the FMR spectra: 1) the shape tigations in the sample MnxSi1-x (x ≈0.52) having the most pro- magnetic anisotropy depending on the form of the sample and 2) the nounced high-TC FM, using microscope SmartSPM (AIST-NT). crystal structure driven magnetic anisotropy which is due to relativistic FMR spectra of the obtained samples were studied at temperatures interactions between electrons and ions in the sample material. T = 4 − 300 K in the wide range of frequencies ( f =7−60GHz) and Following a simplest model used in Ref. [12] it is easy to obtain: magnetic fields (H =0−30kOe). To detect the resonance absorption 2K signal, magnetic field dependences of the microwave power transmitted K MπM=4 + 1 , (2) through the cavity resonator with the sample inside were measured at M ff constant frequency. Measurements were carried out for di erent where K1 is the phenomenological constant of the second-order easy orientations of the magnetic field with respect to the film plane. plane crystal structure driven magnetic anisotropy. In the absence of K1, Eq. (1) transforms into the well known Kittel formula for the 3. Experimental results and discussion applied field H lying in the film plane [13]. According to Eq.
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